1. Field of the Invention
The invention relates to the technical field of low-voltage electric apparatus, more particularly, relates to a control technology of an alternating-current contactor.
2. The Related Art
Most of the electronic control technologies of an alternating current contactor use a control circuit built by a single-chip microcomputer or discrete components as a core, and adopts a PWM (pulse-width modulation) control mode to drive an electromagnet of the alternating current contactor to achieve reliable attraction, holding or releasing. The overall technical parameters shall meet the requirements provided in Standard GB14048.4, and realizes an energy-saving purpose.
The Chinese patent application with the application number CN201210196762.4, the publication number CN102709118A, and entitled “An energy-saving device of an alternating current contactor” discloses an energy-saving device of an alternating current contactor, comprising a peripheral circuit connected with the alternating current contactor and a signal processor connected with the peripheral circuit. The signal processor comprises a comparison circuit, a zero-crossing detection circuit, a pulse width modulation circuit, a selection circuit, a low-voltage detection circuit and a signal synthesis circuit. The energy-saving device disclosed by CN102709118A can only perform adjustable control of the PWM pulse width of small current during a holding stage of the electromagnet under different control voltages. However, during an attraction stage, the PWM pulse width is not adjustable. An attraction action of the electromagnet is driven by a plurality of fixed power frequency pulses, the amplitude and the dynamic characteristics of the attraction current cannot be controlled. Therefore, an attraction characteristic of the contactor is considerably influenced. This energy-saving device is not suitable for the applications which require frequent operations. A service life of the contactor is not good, while energy conservation is not facilitated. In addition, the PWM frequency of the small current for the holding stage is fixed to be a power frequency signal. Though a holding current can be kept as a constant current at a time level of 20 ms, the dynamic variation range of the small current within 20 ms is large, and the working frequency band belongs to a human ear receivable range and cannot further reduce the running noise of the contactor, the service life of the contactor is also influenced. Moreover, an automatic adjustment of the wide range of the control voltage cannot be realized. Therefore, the device cannot be applied to applications with large control voltage fluctuation. Meanwhile, energy-saving devices and coils with different parameters are required to be configured in order to meet the requirements of different control voltages. Thus the product model is increased, the management cost and the warehouse storage goods cost are correspondingly increased.
The Chinese patent application with the application number CN201210530495.X, the publication number CN103021735A, and entitled “alternating current contactor intelligent control module with high-speed pulse width modulation function” discloses an alternating current contactor intelligent control module with a high-speed pulse width modulation function, comprising an AC/DC power supply supplying power for an alternating-current contactor coil through a rectifying and filtering circuit and a power electronic switch. The alternating current contactor intelligent control module further comprises a voltage sampling circuit, a single-chip microcomputer control system, a two-way DA conversion circuit, a sawtooth wave generation circuit, a square wave clock signal, a high-speed pulse width modulator, a bootstrap circuit, a compensation circuit, a switching point detection circuit, a hall current sensor, a follow current circuit, a low voltage holding loop and a constant voltage/constant current automatic switching circuit. An attraction stage and a holding stage of the electromagnet disclosed by CN103021735A adopt two completely different control strategies. During attraction, a current feedback loop detects a coil current through a hall current detector under a PWM control mode, the single-chip microcomputer controls an exciting current of an adjusting coil according to the current feedback signal. When the attraction stage is completed, it is switched to a low-voltage holding loop through a trigger switching point detection circuit, supplying direct power supply to the electromagnet coil during the holding stage to drive the electromagnet. For the purpose of achieving normal operations of the contactor and an energy-saving mode of large attraction current and small holding current, two control loops (an attraction control loop and a holding control loop) are used in the whole control module, and the control mode further includes considerable hardware circuits such as a hall current sensor circuit and a trigger switching point detection circuit, and software expenditures. The cost is increased, more space is occupied. It is not suitable for the applications of small current contactors. The application range of this module is limited. However, for a large current contactor, due to the fact that more energy is required for the holding stage of the coils, a large capacity for sufficient power output is required so that a low-voltage holding circuit can provide sufficient energy to maintain the contact at a holding state, which may further increase the cost.
In addition, according to the existing solutions, a follow current circuit is provided in an electromagnet coil circuit. For a large current contactor, the presence of the follow current circuit will cause a slow change of the coil current because of a large inductance of the coil, which will lengthen a releasing process of the contactor and considerably influence a releasing performance of the contactor.